Models project that increasing atmospheric concentrations
of greenhouse gases will result in changes in daily, seasonal, inter-annual,
and decadal variability. There is projected to be a decrease in
diurnal temperature range in many areas, decrease of daily variability
of surface air temperature in winter, and increased daily variability
in summer in the Northern Hemisphere land areas. Many models project more
El Niño-like mean conditions in the tropical Pacific. There is
no clear agreement concerning changes in frequency or structure of naturally
occurring atmosphere-ocean circulation patterns such as that of the North
Atlantic Oscillation (NAO)

Models project that increasing atmospheric concentrations
of greenhouse gases result in changes in frequency, intensity, and duration
of extreme events, such as more hot days, heat waves, heavy precipitation
events, and fewer cold days. Many of these projected changes would
lead to increased risks of floods and droughts in many regions, and predominantly
adverse impacts on ecological systems, socio-economic sectors, and human
health (see Table SPM-2 for details). High
resolution modeling studies suggest that peak wind and precipitation intensity
of tropical cyclones are likely to increase over some areas. There is insufficient
information on how very small-scale extreme weather phenomena (e.g., thunderstorms,
tornadoes, hail, hailstorms, and lightning) may change.

Greenhouse gas forcing in the 21st century could set in
motion large-scale, high-impact, non-linear, and potentially abrupt changes
in physical and biological systems over the coming decades to millennia,
with a wide range of associated likelihoods.

Some of the projected abrupt/non-linear changes
in physical systems and in the natural sources and sinks of greenhouse
gases could be irreversible, but there is an incomplete understanding
of some of the underlying processes.The
likelihood of the projected changes is expected to increase with the rate,
magnitude, and duration of climate change.

Examples of these types of changes include:

Large climate-induced changes in soils and vegetation may be possible
and could induce further climate change through increased emissions
of greenhouse gases from plants and soil, and changes in surface properties
(e.g., albedo).

Most models project a weakening of the thermohaline circulation of
the oceans resulting in a reduction of heat transport into high latitudes
of Europe, but none show an abrupt shutdown by the end of the 21st century.
However, beyond the year 2100, some models suggest that the thermohaline
circulation could completely, and possibly irreversibly, shut down in
either hemisphere if the change in radiative forcing is large enough
and applied long enough.

The Antarctic ice sheet is likely to increase in mass during the
21st century, but after sustained warming the ice sheet could lose significant
mass and contribute several meters to the projected sea-level rise over
the next 1,000 years.

In contrast to the Antarctic ice sheet, the Greenland ice sheet is
likely to lose mass during the 21st century and contribute a few cm
to sea-level rise. Ice sheets will continue to react to climate warming
and contribute to sea-level rise for thousands of years after climate
has been stabilized. Climate models indicate that the local warming
over Greenland is likely to be one to three times the global average.
Ice sheet models project that a local warming of larger than 3°C,
if sustained for millennia, would lead to virtually a complete melting
of the Greenland ice sheet with a resulting sea-level rise of about
7 m. A local warming of 5.5°C, if sustained for 1,000 years, would
likely result in a contribution from Greenland of about 3 m to sea-level
rise.

Continued warming would increase melting of permafrost in polar,
sub-polar, and mountain regions and would make much of this terrain
vulnerable to subsidence and landslides which affect infrastructure,
water courses, and wetland ecosystems.

Projected Changes during the
21st Century in Extreme Climate Phenomena and their Likelihood

Representative Examples of
Projected Impactsa
(all high confidence of occurrence in some areas)

Higher maximum temperatures, more hot days and heat
wavesb over nearly all land areas (very likely)

Increased incidence of death and serious illness in
older age groups and urban poor.
Increased heat stress in livestock and wildlife.
Shift in tourist destinations.
Increased risk of damage to a number of crops.
Increased electric cooling demand and reduced energy supply reliability.

Decreased cold-related human morbidity and mortality.
Decreased risk of damage to a number of crops, and increased risk
to others.
Extended range and activity of some pest and disease vectors.
Reduced heating energy demand.

Increased risks to human life, risk of infectious
disease epidemics and many other risks.
Increased coastal erosion and damage to coastal buildings and infrastructure.
Increased damage to coastal ecosystems such as coral reefs and mangroves.

Intensified droughts and floods associated with El
Niño events in many different regions (likely) (see
also under droughts and intense precipitation events)

a. These impacts can
be lessened by appropriate response measures.b.
Information from WGI
TAR Technical Summary (Section F.5).c. Changes
in regional distribution of tropical cyclones are possible but have
not been established.

Changes in climate could increase the risk of abrupt
and non-linear changes in many ecosystems, which would affect their function,
biodiversity, and productivity. The greater the magnitude and rate
of the change, the greater the risk of adverse impacts. For example:

Changes in disturbance regimes and shifts in the location of suitable
climatically defined habitats may lead to abrupt breakdown of terrestrial
and marine ecosystems with significant changes in composition and function
and increased risk of extinctions.

Sustained increases in water temperatures of as little as 1°C,
alone or in combination with any of several stresses (e.g., excessive
pollution and siltation), can lead to corals ejecting their algae (coral
bleaching) and the eventual death of some corals.

Temperature increase beyond a threshold, which varies by crop and
variety, can affect key development stages of some crops (e.g., spikelet
sterility in rice, loss of pollen viability in maize, tubers development
in potatoes) and thus the crop yields. Yield losses in these crops can
be severe if temperatures exceed critical limits for even short periods.